Waters contaminated with salt can be unusable or hazardous to the environment. For example, mines use freshwater and discharge tailings into ponds. Tailings water is commonly about 99.8% freshwater by mass, but is unusable due to low levels of salts, for example 0.1 to 0.2% by mass. Desalination is being increasingly used in both industries as regulations require treatment of impaired water. Desalination is also used in coastal regions to produce freshwater from seawater, with the more saline brine reject returned to the ocean. Reverse osmosis is the dominant desalination technology, but faces osmotic pressure and fouling limits that reduce production of freshwater per unit of input water. Conventional thermal desalination systems do not experience osmotic pressure limits, but instead face heat transfer surface fouling issues at higher temperatures due to the inverse temperature solubility of calcium sulfate and carbonates.
In humidification-dehumidification (HDH) desalination systems an air stream is humidified by warm saltwater, with the air stream having a wet bulb temperature lower than the warm saltwater temperature. The warm saltwater drips through fill material to promote heat and mass transfer from the warm saltwater to the air stream. The humidified air stream is then cooled by a heat exchanger surface that is colder than the wet bulb temperature of the humidified air. As the humidified air is cooled, the air's ability to hold vapour decreases and moisture condenses on the cooler heat exchanger tubes. A cool liquid or evaporating refrigerant flows internal to the heat exchanger tubes.
Air based HDH systems offer certain advantages as well as drawbacks over conventional thermal desalination systems. One advantage is that HDH systems operate at lower temperature, for example less than 80° C., reducing fouling and enabling the use of waste heat to run the process. A lower operation temperature also allows the HDH apparatus to be constructed from less expensive and corrosion resilient engineered plastics such as high density polyethylene (HDPE) or polyvinyl chloride (PVC), which are less prone to fouling. One of the drawbacks of HDH systems is that the footprint of a conventional HDH apparatus is generally higher than a conventional thermal desalination apparatus which employs higher pressure and temperature steam with a lower specific volume. Since a conventional HDH apparatus is operated at a lower temperature, the water vapour has a higher specific volume necessitating more vapour space, hence a larger footprint. It would therefore be of benefit to provide a HDH system that provides such vapour space at a low cost. Lower cost can be achieved through modularization, improved repeatability, economies of scale, and reduction of site work.